Typically in continuous inkjet printers, a pressurized ink is formed into continuous inkjet filaments projecting from multiple ink discharge nozzles in a printhead. Filament stimulation sources such as ink heaters or transducers operate as ink droplet generators each time they are activated, by causing filament end-lengths to be broken off at the respective nozzles to provide discrete ink droplets which, in turn, are deposited on a print medium moving relative to the printhead. The interval between successive droplet break-offs at any one nozzle matches the interval between successive activations of the filament stimulation source for that nozzle. The longer the interval between successive activations of the filament stimulation source for the nozzle, the longer the opportunity for the continuous inkjet filament to increase lengthwise at the nozzle and the larger the ink droplet. Conversely, the shorter the interval between successive activations of the filament stimulation source for the nozzle, the shorter the opportunity for the continuous inkjet filament to increase lengthwise at the nozzle and the smaller the ink droplet. Thus, the volume of the ink droplet, when a droplet break-off occurs at the nozzle, corresponds to the frequency of activation of the filament stimulation source for the nozzle.
Successive ink droplets can be altered between printing and non-printing trajectories or paths. Those ink droplets that are in a printing trajectory are allowed to reach the print medium. Those ink droplets that are in a non-printing trajectory can be collected in a ink gutter or catcher and then recycled.
A problem that exists is that dirt or dried ink can accumulate on a nozzle, particularly in the region where the continuous inkjet filament projects from the nozzle. When this occurs, the nozzle must be considered defective because the ink droplets that result from filament end-lengths being broken off at the nozzle may be misdirected with respect to the printing trajectory that the ink droplets should take. Consequently, the printed image may be of a lesser quality.
The problem of misdirected ink droplets is particularly acute in continuous inkjet printers because ink flow to form a continuous inkjet filament at a nozzle that is defective cannot be stopped.